In recent years, image forming apparatuses such as full-color copying machines have been developed and widely used. Since these image forming apparatuses produce highly precise reproductions of original images, they may be used for the crime of counterfeiting paper money and securities, for example. In order to prevent such a crime, the following apparatuses have been developed. These apparatuses, for instance, stop copying operations when a copying-prohibited document such as paper money is detected, and prints a particular mark, which has been given to a copying machine, on a copy of the document image so that the copying machine used for producing the copy is identified from the mark on the copy.
For example, apparatuses performing the former function are disclosed in the following documents. Japanese Publication for Unexamined Patent Application No. 83571/1990 and No. 285978/1989 teach apparatuses which store patterns of copying-prohibited documents such as paper money and become inoperable when a document to be copied has one of the patterns stored. Japanese Publication for Unexamined Patent Application No. 205272/1992 and No. 316783/1989 disclose apparatuses which, if a document to be copied is detected as paper money, prints the whole portion corresponding to paper money in deep black or halftone when outputting a copy of the document. The apparatuses which perform the latter function are disclosed in the following documents. Japanese Publication for Unexamined Patent Application No. 294682/1992 and No. 302267/1992 teach apparatuses which print a pattern unique to a copying machine in unnoticeable color or a unique feature of the copying machine, for example, the product number when outputting a copy of the document image.
However, although the above-mentioned conventional structures can prevent a crime, it is impossible to effectively identify the person who committed the socially punishable crime, such as counterfeiting paper money. Hence, the conventional apparatuses can not perfectly prevent counterfeiting of paper money.